.TH std::regex_traits::lookup_classname 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::regex_traits::lookup_classname \- std::regex_traits::lookup_classname

.SH Synopsis
   template< class ForwardIt >

   char_class_type lookup_classname( ForwardIt first,
                                     ForwardIt last,

                                     bool icase = false ) const;

   If the character sequence [first, last) represents the name of a valid character
   class in the currently imbued locale (that is, the string between [: and :] in
   regular expressions), returns the implementation-defined value representing this
   character class. Otherwise, returns zero.

   If the parameter icase is true, the character class ignores character case, e.g. the
   regex [:lower:] with std::regex_constants::icase generates a call to
   std::regex_traits<>::lookup_classname() with [first, last) indicating the string
   "lower" and icase == true. This call returns the same bitmask as the call generated
   by the regex [:alpha:] with icase == false.

   The following narrow and wide character class names are always recognized by
   std::regex_traits<char> and std::regex_traits<wchar_t> respectively, and the
   classifications returned (with icase == false) correspond to the matching
   classifications obtained by the std::ctype facet of the imbued locale, as follows:

                    Character class name     std::ctype classification
                    Narrow        Wide
                  "alnum"     L"alnum"     std::ctype_base::alnum
                  "alpha"     L"alpha"     std::ctype_base::alpha
                  "blank"     L"blank"     std::ctype_base::blank
                  "cntrl"     L"cntrl"     std::ctype_base::cntrl
                  "digit"     L"digit"     std::ctype_base::digit
                  "graph"     L"graph"     std::ctype_base::graph
                  "lower"     L"lower"     std::ctype_base::lower
                  "print"     L"print"     std::ctype_base::print
                  "punct"     L"punct"     std::ctype_base::punct
                  "space"     L"space"     std::ctype_base::space
                  "upper"     L"upper"     std::ctype_base::upper
                  "xdigit"    L"xdigit"    std::ctype_base::xdigit
                  "d"         L"d"         std::ctype_base::digit
                  "s"         L"s"         std::ctype_base::space
                  "w"         L"w"         std::ctype_base::alnum
                                           with '_' optionally added

   The classification returned for the string "w" may be exactly the same as "alnum",
   in which case isctype() adds '_' explicitly.

   Additional classifications such as "jdigit" or "jkanji" may be provided by
   system-supplied locales (in which case they are also accessible through
   std::wctype).

.SH Parameters

   first, last - a pair of iterators which determines the sequence of characters that
                 represents a name of a character class
   icase       - if true, ignores the upper/lower case distinction in the character
                 classification
.SH Type requirements
   -
   ForwardIt must meet the requirements of LegacyForwardIterator.

.SH Return value

   The bitmask representing the character classification determined by the given
   character class, or char_class_type() if the class is unknown.

.SH Example

   Demonstrates a custom regex traits implementation of lookup_classname() / isctype():


// Run this code

 #include <cwctype>
 #include <iostream>
 #include <locale>
 #include <regex>

 // This custom regex traits uses wctype/iswctype to implement lookup_classname/isctype.
 struct wctype_traits : std::regex_traits<wchar_t>
 {
     using char_class_type = std::wctype_t;

     template<class It>
     char_class_type lookup_classname(It first, It last, bool = false) const
     {
         return std::wctype(std::string(first, last).c_str());
     }

     bool isctype(wchar_t c, char_class_type f) const
     {
         return std::iswctype(c, f);
     }
 };

 int main()
 {
     std::locale::global(std::locale("ja_JP.utf8"));
     std::wcout.sync_with_stdio(false);
     std::wcout.imbue(std::locale());

     std::wsmatch m;
     std::wstring in = L"風の谷のナウシカ";
     // matches all characters (they are classified as alnum)
     std::regex_search(in, m, std::wregex(L"([[:alnum:]]+)"));
     std::wcout << "alnums: " << m[1] << '\\n'; // prints "風の谷のナウシカ"
     // matches only the katakana
     std::regex_search(in, m,
                       std::basic_regex<wchar_t, wctype_traits>(L"([[:jkata:]]+)"));
     std::wcout << "katakana: " << m[1] << '\\n'; // prints "ナウシカ"
 }

.SH Output:

 alnums: 風の谷のナウシカ
 katakana: ナウシカ

.SH See also

   isctype indicates membership in a character class
           \fI(public member function)\fP
   wctype  looks up a character classification category in the current C locale
           \fI(function)\fP
